1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to power control wiring devices.
2. Technical Background
Power control devices allow a user to adjust the amount of current delivered to an electrical load, such as a light or a motor. When the electric load is a lighting device, the power control device is commonly referred to as a dimmer. If the power control device is configured to control a motor, such as a fan motor, the power control device is referred to as a motor speed controller. Motor speed controllers are also used to control the speed of machinery such as power tools, electric drills, chair lifts, stationary machinery, and other such variable speed motor driven elements.
The core component of the power control device is commonly referred to as a series pass element. The amount of current provided by the series pass element is varied by a user-actuated switch mechanism. The switch mechanism may be a continuously variable switch or a selector switch mechanism that selects from a predetermined number of discrete switch settings. The series pass element may be implemented using a solid state switch. The active switching element in a solid state switch may be a transistor, a MOSFET device, a gate turn-off device, or a thyristor device, such as silicon controlled rectifier (SCRs) or a triac device. When the series pass element in a fan speed control device is a variable impedance, the power control device is commonly referred to as a “dehummer.”
As those of ordinary skill in the art will appreciate, power control devices are typically packaged in a wiring device form factor for installation in an outlet box. Of course, one or more of the power control devices described above may be disposed within the device housing. A unit equipped with both a fan motor and a lighting element, for example, may be controlled by a wiring device that includes both a dimmer and a fan speed control. The exterior of the wiring device includes either screw terminals or wire terminals for subsequent connection between the AC power source and the load. The wiring device form factor also provides a user accessible front face that is includes one or more switch mechanisms such as levers, dials, slide switches, and other such input control mechanisms that permit a user to vary the power to a load.
Prior to device installation, wiring from the AC power source and wiring to the load(s) are disposed inside the outlet box. The outlet box is usually located proximate to the load being controlled. The device is installed by connecting the wiring inside the outlet box to the appropriate wiring device terminals disposed on the exterior of the wiring device. The power control wiring device is then inserted into the outlet box and attached to the outlet box using one or more fasteners. A cover plate is installed to complete the installation.
Some of the drawbacks associated with conventional power control devices are illustrated by referring to FIG. 1. Conventional device 10 includes a dimmer control knob 12 and switch 14 disposed in a protective frame 16. The protective frame 16 is coupled to mounting strap 20. The frame 16 functions as an alignment mechanism for the cover plate (not shown). The frame 16 extends through the cover plate opening when the device installation is complete. The mounting strap 20 is hidden behind the cover plate. The series pass element is typically implemented using a solid state device such as a triac. The mounting strap is then coupled to the triac and functions as a heat sink.
FIG. 2 shows a top view of the conventional mounting strap depicted in FIG. 1. Mounting strap 20 includes two major portions. The first portion is an exterior perimeter formed by tabs 22 and mounting end portions 28. The exterior perimeter surrounds an interior portion 26. One disadvantageous aspect of this design relates to the fact that the exterior perimeter portion (22, 28) and the interior portion 26 are not coplanar. Interior portion 26 is stepped downwardly to create space for the control knob 12 and switch 14. Interior portion 26 serves to thermally isolate mounting strap 20 from front surfaces of the power control device 10. The mounting strap is scored along lines 24. The lines 24 allow installers to repeatedly bend and break off tabs 22 with a pair of pliers if the control device is installed in a multi-gang wall box adjacent to other wiring devices.
One drawback associated with this approach is that the tabs 22 are only connected to end tabs 28 and cannot be connected to interior portion 26 because the design is not coplanar. The air gap between tabs 22 and interior portion 26 represents a major thermal discontinuity which greatly limits the thermal conductivity of the heat sink. Accordingly, the tabs afford minimal heat sinking benefit. Another drawback to this approach relates to the distance between wiring devices mounted in a multi-gang wall box being typically quite small. Tab removal allows the control device to fit in a multi-gang wall box but is disadvantageous because it diminishes the device's heat sinking capabilities. As a result, the power control device must be operated at a de-rated current level in order not to overheat the triac. The above scenario points to another shortcoming of conventional devices. Most conventional power control devices are functionally limited such that multi-gang wall boxes accommodate the assortment of wiring devices required to meet the user's functional requirements.
Another shortcoming of the conventional design also relates to the bi-level design of the mounting strap. In particular, the interior portion 26 encroaches into the interior volume behind the mounting strap 20 that is normally reserved for component placement necessitating an increase in the overall depth of the device. A thicker device is more difficult to install because there is less room in the electrical box for wiring and wire connections.
The heat sink also functions as the device mounting strap. Thus, another issue related to conventional power device heat sinks relates to its connection to the ground terminal. Many devices are wire terminal devices coupled to ground by using a ground wire. One drawback to this approach is that the wire connection to the heat sink occupies too much space within the device, making the over-all device form factor too bulky. Devices that employ ground screw terminals have other issues that are similar to some of the issues discussed above. One approach in forming a ground terminal is to bend one of the tabs 22 (or portion thereof) downwardly to form a screw terminal. A drawback associated with this approach is that the tab would lie outside the outlet box. Another approach is to bend a section of an end portion 28 downwardly such as along dotted line 25 so that the tab lies inside the outlet box. Of course, a tab 22 would have to be eliminated. A drawback to this approach relates to the fact that the over-all surface area used for device cooling is reduced. Yet another drawback related to the ground connection relates to the conduction of too much heat through the ground connection. If the temperature of the ground terminal is excessive, the ground wire insulation or other insulation in vicinity of the ground wire may be compromised, and a fire hazard may be created.
What is needed is a power control device equipped with a heat sink wherein the side tabs do not diminish the thermal conduction capabilities of the heat sink. A planar heat sink having an increased cross-sectional are is also needed to provide improved thermal conduction. Finally, a power control device is needed that has a thermally efficient screw terminal connection that eliminates the drawbacks associated with conventional designs.